Performance Modeling and Benchmarking of Event-Based ... - DVS

64 CHAPTER 4. PERFORMANCE ENGINEERING OF EVENT-BASED SYSTEMS
used a single color (Trigger) to model the pull attempt of the consumer and its resources. In
this pattern, we use two colors to model these aspects:
1. Trigger: responsible to trigger pull attempt.
2. Slot: represents available resources of consumer.
The Trigger token controls if and when the consumer establishes a connection to the broker.
The pull attempt of the consumer is modeled by transition T2 : if an Event, a Trigger and a
Slot (standing for free resources) exist, the consumer pulls an Event. The consumer tries to pull
Events (by firing transition T2 ) until no further resources (Slots) or Events are available in the
depository of the Store respectively the Broker:
• If no further resources are available, the consumer disconnects and waits for resources to
be released (see transition T3 ) before establishing a new connection.
• If there are free resources (Slot tokens) available but no further Event token available, the
consumer closes the connection and waits for ceratin time interval before he reconnects
(see transitions T4 & T5 ).
All places and transitions are illustrated in Figure 4.16. The number of initial Slot tokens
in the ordinary place Store defines, how many events the consumer is able to handle in parallel,
e.g., the number of service places.
QPN Definition
Places:
Place Type Description
Producer S Publishes an event.
Broker S Stores incoming events.
Timer Q Timer queue (scheduling strategy: infinite server).
Store O Stores trigger and slot tokens. Used to model free resources.
Consumer S Consumes events.